​My lab is interested in understanding mechanisms used by the brain to process olfactory cues. We focus on two structures, the olfactory bulb and the piriform cortex, asking basic questions about what neurons are present, how they are connected, and how groups of neurons work to effect a particular output for a circuit. Methodologically, we combine electrophysiological and optical recordings in brain slices, confocal microscopy, as well as computational and ultrastructural approaches. We also use transgenic/viral techniques for labeling specific cell-types and optogenetic manipulation.

Structure and function of glomeruli in the olfactory bulb

One specific current research interest in the lab is in the neuropil structures that line the outer surface of the olfactory bulb, called glomeruli. Glomeruli are the site of input into the bulb from axons of olfactory sensory neurons (OSNs). These structures are an especially attractive model for understanding general circuit function in the brain, as they are quite compact, about 80 µm in diameter, and also have a dedicated population of cells with apical dendritic arbors restricted to one glomerulus. These include ~25 mitral cells, 50 tufted cells, and ~500 GABAergic periglomerular (PG) cells. A glomerulus also has a clear link to function, as each glomerulus codes for information about one odorant receptor in the nose.

In studies of the organization of synapses in glomeruli, we recently obtained one surprising finding that went against the conventional view about how OSNs signal to mitral cells. In experiments in which recordings were made simultaneously from tufted cells and mitral cells affiliated with the same glomerulus (see Figure), we found that tufted cells, but not mitral cells, displayed significant electrical signals reflecting direct transmission from OSNs. Such results have led us to propose that OSNs mainly communicate to mitral cells through a multi-step path in which tufted cells act as intermediaries. We are now attempting to understand the mechanistic basis for the weak OSN signaling onto mitral cells – is it that OSN synapses are rare or more subtle effects related to the location of the synapses with respect to shunting conductances? – while we are also trying to understand the functional relevance of these mechanisms. One possible outcome of having OSNs signal directly to tufted cells, but not mitral cells, is that the mitral cell output is much more heavily regulated than that of tufted cells.

Interactions between olfactory bulb and cortex

In other studies, we are examining interactions between the olfactory bulb and downstream neurons in the piriform cortex. For example, we are testing what components of the piriform cortex circuit allow it to be tuned to specific output features of the bulb, including synchronized activity in mitral cells. Also, what is the function of cortical projections that extend back to the bulb? In these latter studies, we employ optogenetic methods to achieve selective activation of feedback projections in the bulb.